The alias is used similar to the add-on domain. Domain aliasing is creating additional domain names that would point to the IP of a different domain. For example, if your domain name is test.com, you can register another domain name, e.g. test.net and have it point to the location of test.com
In simple words, it is just another name for some mailbox and all emails are redirected into the mailbox for which you created an alias.
